Radiant exitance (or emittance) is a term of radiometry and is defined as the radiant flux (optical power = energy per unit time) which is emitted by some surface (e.g. of a light source) per unit area.
In the SI system, the units of the radiant exitance are W/m2 (watts per square meter). The related term irradiance has the same units, but applies to received radiation.
A related quantity is the spectral exitance, which is the exitance per unit frequency or wavelength interval. It has units of W / (m2 Hz) or W / (m2 nm), for example.
Example: Radiant Exitance of Blackbody Radiation
from which one can calculate the spectral exitance by integration over all solid angles of a hemisphere:
If this is integrated over all frequencies, one obtains the Stefan–Boltzmann law for the radiant exitance of thermal radiation of a black body:
with the Stefan–Boltzmann constant σ ≈ 5.6704 · 10−8 W m−2 K−4.